How are endosymbionts spreading from one ecological community to
another?
Ecological communities are a diverse assemblage of many different
species involved in a web of interactions with each other (Agrawal et
al., 2007). However, rarely, are such communities isolated from each
other. There are certain members which are relatively cosmopolitan and
interconnect with members of multiple communities (Stireman & Singer,
2003) leading to a metacommunity-wide distribution (Brown et al., 2020).
The distribution of these arthropods can lead to horizontal transfer of
their resident endosymbionts to distinct ecological communities. Within
the soil arthropod community, we have found one such example which can
potentially be a source of horizontal transfer of endosymbionts across
many other communities. The macropterous form of the planthopperNilaparvata lugens (morph0111, BOLD ID SAEVG089-20, Table S3) was
found from leaf litter sampling. N . lugens is a highly
destructive pest of rice across tropical Asia and can also survive on
other tropical grass species (Khan, Saxena, & Rueda, 1988). It is known
to migrate long distances in search of actively growing rice plants
(Riley, Smith, & Reynolds, 2003). The presence of N .lugens is unsurprising as our sampling season (October) coincided
with the rice harvesting season in North India. N . lugensis known to be infected with several endosymbionts like Wolbachiaand Arsenophnonus (Qu et al., 2013). In the present study, it was
found to be infected with Wolbachia ST-163 from the B supergroup.
The same Wolbachia sequence type has also been reported fromN . lugens from Southern China (Zhang, Han, & Hong, 2013).
This indicates that such invasive pest species can potentially introduce
their resident endosymbionts into many different arthropod communities.
Conversely, the presence of very similar endosymbionts in geographically
distinct locations can indicate their spread from one ecological
community to other. The Wolbachia B supergroup, ST-41, has been
detected from a phorid fly (morph0285) in our dataset. The same ST-41
has been found in calyptrate flies (Stahlhut et al., 2010) as well as
from several other lepidopterans (Ilinsky & Kosterin, 2017; Narita et
al., 2011; Russell et al., 2009; Salunke et al., 2012). This is not
unexpected given the diversity of Wolbachia infections. However,
what is unexpected is the location of the hosts with ST-41 ranges from
North America, Africa, Russia, South and South-Eastern Asia all the way
to Japan. Unfortunately, it is difficult to conjecture about the reasons
behind such a huge range, as corroborating community-wide data is
lacking.
The above two instances testify to the utility of a MLST based approach
to understand Wolbachia diversity and spread across global
arthropod communities. Moreover, these cases also highlight the
importance of collecting community-wide data to understand the probable
chain of transfer of these bacteria. Such data can also illuminate
similar connections for the spread of Arsenophonus andCardinium if employed with multi-locus data (Jousselin, Cœur
d’Acier, Vanlerberghe‐Masutti, & Duron, 2013; Stouthamer, Kelly, Mann,
Schmitz-Esser, & Hunter, 2019).
A major goal of endosymbiont research is to explain the tempo and mode
of their spread across arthropod communities across the world. We
contend that evaluating endosymbiont diversity within specific
ecological communities is the key to understand this spread. Such
studies would give us specific examples of bacterial strains that are
better at spreading as well as uncover specific ecological roles of
arthropod hosts which are more amenable to horizontal transfer of their
resident endosymbionts. As data from such studies accumulate higher
level patterns will emerge which can then be empirically tested.